The present invention relates in general to sensors, and in particular to a new and useful acoustic emission sensor suitable for use in hostile environments. According to the present invention, a high-frequency fiber optic vibration sensor is adapted to acoustic emission sensing in hostile high-temperature environments, high EMI environments, and applications where explosion hazards exist.
Presently, piezoelectric transducers of the lead zirconate titanate (PZT) type are used as detectors in commercial systems such as acoustic leak locators on boilers. These leak locators can detect and analyze acoustic energy that results when high-pressure steam exits a pin-hole leak in a steam generator tube, and can distinguish between a leaking tube and an operating sootblower. Since the operating temperature range of the PZT acoustic detector is limited to 150.degree. C., an acoustic waveguide must be used to transfer acoustic energy out of the boiler. The waveguide introduces a loss factor of about ten into the detection system, however. See for example, U.S. Pat. Nos. 4,858,462 and 5,351,655.
An optical fiber modulator is disclosed in U.S. Pat. No. 4,810,051. The use of optical fibers for measuring pressure or strain is also disclosed in U.S. Pat. No. 4,770,492. See U.S. Pat. No. 4,678,903 for a self-aligning fiber optic microbend sensor.
A temperature-compensating fiber optic strain sensor is disclosed in U.S. Pat. No. 5,345,519. A pressure system using an optical resonator cavity is disclosed in U.S. Pat. No. 4,933,545. Also see U.S. Pat. No. 4,928,004 for a method and apparatus for measuring strain, utilizing optical fibers. A fiber optic acoustic sensor is disclosed in U.S. Pat. No. 4,162,397. Also see U.S. Pat. No. 4,071,753.
U.S. Pat. No. 4,950,886 to Claus et al. discloses a partially reflecting optical fiber splice for temperature and strain measurement. A particular feature of the '886 patent is that it avoids the need to rely upon microbends and instead uses measurement techniques that correspond to changes in an air gap between the fibers in a particular splice. A system of optical fibers and splices can be located throughout a structure to yield an array of strain and temperature measurements.
U.S. Pat. No. 5,301,001 to Murphy et al. discloses extrinsic fiber optic displacement sensors and displacement sensing systems. The patent relates generally to fiber optic interferometric sensors and, more particularly, to extrinsic Fizeau interferometric fiber optic sensors having a particular application in hostile environments to dynamically monitor strain, temperature or pressure in mechanical structures. As used therein, "strain" is defined to mean strain, temperature, pressure, magnetic fields, and other like phenomena that can be translated into a displacement depending upon the application. Further, the inventors characterized known Fabry-Perot interferometers as having multiple reflections within a cavity, while Fizeau interferometers were said to operate on the principle of a single reflection within the cavity.
U.S. Pat. No. 5,202,939 to Belleville et al. disclosed a Fabry-Perot optical sensing device for measuring a physical parameter such as pressure, temperature, the refractive index, and especially strain in or deformation of a body. A Fabry-Perot interferometer is optically coupled to a wedge-shaped Fizeau interferometer cavity to produce a spatially-spread light signal indicative of the transmittance or reflectance properties of the Fabry-Perot interferometer, the light signal being indicative of the parameter being sensed.
U.S. Pat. No. 5,189,299 discloses an apparatus and method for sensing strain in a waveguide. A comprehensive description of a Fabry Perot Interferometer (EFPI) can be found in an article by Murphy, et al., found in the Proceedings of the 1993 ASME Winter Annual Meeting, New Orleans, La., "Acoustic Wave Response of the Extrinsic Fabry-Perot Interferometer (EFPI) Optical Fiber Sensor", available in a publication, Adaptive Structures and Material Systems, Aerospace Division American Society of Mechanical Engineers, at AD-Vol. 35, pp 395-399, published by ASME, New York, N.Y.